In a PET apparatus, numerous detectors first detect pair annihilation photons of 511 keV which are positrons generating from a patient administered with a positron-emitting radionuclide, combining with adjacent electrons and emitted in directions of 180 degrees. When two detectors detect photons within a definite period of time, they are counted as one pair of annihilation photons, and it is determined that a pair annihilation generating point exists on a straight line linking the detector pair having detected them. Such coincidence information is accumulated, and an image reconstruction process is carried out to create a positron-emitting radionuclide distribution image (RI distribution image).
However, when scattered pair annihilation photons are counted as a coincidence (hereinafter also called “scatter coincidence”), a true pair annihilation generating point is not on the line linking the detector pair having detected them. The scatter coincidence, therefore, lowers the image quality of a reconstruction image. Thus, image quality is improved by estimating scattered components using measured photon energy information, and deducting them when carrying out a reconstruction process (see Patent Documents 1-3, for example).
[Patent Document 1]                Unexamined Patent Publication H7-113873        
[Patent Document 2]                Unexamined Patent Publication No. 2001-356172        
[Patent Document 3]                Unexamined Patent Publication No. 2000-28727        